That is the result he also got using (pi) - (theta). However, for me, it visual doesn't make sense. The angle of the center triangle must (in my eyes at least) (pi) - (2theta). Is the supplementary angle different?

Edit: Looking at again, it looks like the second solution includes the first angle as well which would make sense. Now I don't understand the supplement angle. Checking Google as of now...

angle between

Originally Posted by Altermeris

That is the result he also got using (pi) - (theta). However, for me, it visual doesn't make sense. The angle of the center triangle must (in my eyes at least) (pi) - (2theta). Is the supplementary angle different?